The present invention includes a control system for regulating the relative positions of two seal members in a mechanical seal assembly to achieve optimum seal performance. The control system of the invention examines a predetermined parameter of the seal assembly when operating and, when a given phenomenon (signifying a reference position of the two seal faces) is detected through examination of that parameter, the relative positions of the seal members are adjusted for optimum seal performance.
A mechanical seal assembly is generally comprised of two separate seal faces, each of which has a smooth radial face surface opposed to a corresponding radial face surface of the other seal element. These two seal elements are spaced apart by a very minute distance, of the order of 50 to 200 microinches, to minimize leakage of a fluid under high pressure from a cavity within a fixed housing, past the opposed face surfaces into a low pressure space adjacent a shaft rotating in the housing. One of the seal elements is generally fixed to, or with respect to, the housing, and the other seal face is positioned adjacent, and biased toward, the fixed seal face by a spring, bellows, or other suitable member. The various operating conditions or parameters of the entire assembly change significantly as the system starts from rest, with the shaft not rotating, and is thereafter accelerated up to an operating condition where it might run for hundreds of hours before the system is again de-energized. Under such operation the parameters such as fluid temperature, pressure, angular speed of the shaft, thickness of the film (or gap between the seal faces), and viscosity or other characteristics of the fluid, may change. The fixed face can not move in the axial direction, parallel to the shaft, and the floating face (the one biased toward the fixed face) moves slightly in an attempt to accommodate the fluid dynamic and mechanical forces during system operation. While the seal faces appear very flat to the naked eye, when magnified thousands of times it is evident there are considerable asperities or irregularities on each surface. The thickness of the film in the gap (between faces) should exceed the roughness asperity height of the faces to prevent mechanical contact between the seal elements, but must not be too wide or excessive leakage will occur. The firm thickness for any one seal depends both on initial seal design (configuration, the materials from which the seal members are constructed, and so forth), and on the actual operating conditions (particularly fluid to be be sealed, shaft velocity, temperature and pressure). Accordingly, a seal assembly designed for one particular set of operating conditions may leak excessively or even fail due to mechanical contact after operating conditions are changed, such as by using a different fluid, changing the shaft speed, or changing some other condition.
For many mechanical seal assemblies, the film thickness (or gap width) is determined by the precise geometry of the opposed seal face surfaces. By way of example, if the two seal faces are perfectly flat and parallel each other, so that the gap between them is of a uniform width across the entire extent of the radial seal faces, a film cannot be maintained and it will collapse, allowing mechanical contact between the faces with resultant wear and possible destruction of the seal. In order to maintain a finite film thickness, it has been found that the gap between the adjacent faces must converge in the radial direction, going from the point farther from the shaft center (the high pressure side of the seal assembly) radially inwardly to the low pressure side, nearer the shaft axis. An alternate configuration could have the high pressure at the shaft and the low pressure at the outside diameter. In this case the adjacent faces must diverge in the radial direction, going from the point farther from the shaft center (low pressure side of the seal assembly) radially inwardly to the high pressure side nearer the shaft axis. Furthermore the greater the degree of convergence, the larger is the film thickness. Since the general practice has been to manufacture mechanical seal faces to be initially flat and parallel, such face convergence occurs (at least for commercially successful seal assemblies) as a result of thermal and mechanical deformations of the seal faces and other components of the seal assembly during operation. It should be noted that the required film thicknesses are so small that the corresponding necessary deformations are also very small, of the order of 20 to 100 microinches. Further once a conventional seal has been designed, built to specifications and then placed in service, the deformations under operating conditions (and hence the resultant film thickness) cannot be controlled or changed at will. Rather the deformation, and thus the ultimate film thickness, is determined not only by the original design and manufacture, but also by the subsequent operating conditions. A complete teaching of the best mode known for controlling convergence between adjacent seal faces is set out in another patent application entitled "Controllable Mechanical Seal", filed of even date, having Ser. No. 840,369 and assigned to the assignee of this application. The teaching of that copending application is hereby incorporated by reference in this application. That application issued Feb. 17, 1987 as U.S. Pat. No. 4,643,437.
In that Patent seal face convergence is controlled after sensing some parameter in the operation of the mechanical seal assembly. Such a parameter can be temperature, pressure, proximity of the seal faces, or some other useful information. Various attempts have been made to select and utilize different parameters, but no optimum system has been found to regulate the positions, much less the convergence, of the two adjacent seal faces under dynamic operating conditions.
It is therefore a primary consideration of the present invention to provide a control system for regulating the positions of the seal faces in a mechanical seal assembly, which system reacts dynamically to real time changes in the seal operating conditions and effects a regulation to provide not only minimal wear of the seal faces but also minimum leakage across the seal faces.
A more particular consideration is to produce an adaptive control system which incorporates a "self-learning" method to adapt automatically to arbitrary changes in fluid medium temperature or pressure, and to shaft rotation speed.